The present invention relates to a method of producing distributing reflectors, primarily for tuneable lasers, and also to reflectors produced in accordance with the method.
Tuneable semiconductor lasers include several sections through which current is injected, these sections typically being three or four in number. The wavelength, power and mode purity of the lasers can be controlled by regulating the current injected into the various sections. Mode purity implies that the laser is at an operation point, i.e. at a distance from a combination of the drive currents where so-called mode jumps take place and where lasering is stable and side mode suppression is high.
Special requirements are placed on different applications with respect to controlling wavelength. In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over long periods of time after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.
The distributed Bragg reflector (DBR) has played a very significant part in the development of modern semiconductor lasers. The distributed Bragg reflector enables the selection of a narrow wavelength range, therewith enabling a single longitudinal oscillation mode to dominate strongly, which means, in turn, that the spectral width of the laser light will be very small. A multiple wavelength reflector can be created, by modifying a DBR. Any one of these wavelengths can be selected with the aid of a spectral selection mechanism, therewith producing the basis of a highly tuneable laser. There are many reasons for using tuneable lasers in wavelength-multiplexed optical networks, for instance. One use is as a backup laser for a number of other lasers of fixed wavelength. The telecommunications market places high requirements on the components used. A tuneable laser must therefore provide a comparable alternative to a laser of fixed wavelength. The reflector is a very important part of the laser, where enhanced reflectance is highly significant to component performance.
It is known to construct a grating reflector with multiple reflection peaks. The Fourier relationship between, the perturbation of the waveguide and the reflection spectrum has been used as a tool in this respect. A sample distributed Bragg reflector can therewith be identified as a possible way of achieving simultaneous reflections of a number of narrow peaks and different wavelengths.
The use of frequency-modulated uniform gratings to create a suitable reflector design has also been proposed. A reflector having multiple wavelengths is obtained by producing a series of two or more identical frequency modulated gratings. A reflector of this kind is called a superstructure grating (SSG).
Another method referred to as the binary superimposed grating (BSG) has recently been presented. This method lacks the SSG superstructure.
In the BSG method, the magnitude of a design function is used to determine whether or not the waveguide shall include a material having a low index or a material having a high index.
The contribution of this method to known technology lies in the binary approach with a constant s, where s is a distance in which the refractive index has a constantly high or low value, and also in the selection mechanism for a high and a low index.
The method implies the use of a grating that includes a number of grooves or lines where the width of the grooves varies between different grooves and where all grooves have a width which is a multiple of s, and where the distance between mutually adjacent grooves also varies with a multiple of s. Thus, both groove width and groove interspacing vary.
This variation in groove width makes manufacture difficult.
An object of the present invention is to enable a grating of this kind to be produced in a much simpler way.
Accordingly, the present invention relates to a method of producing a distributed reflector which includes a grating and which is a multiple wavelength reflector, and in which method the grating is provided with regions (21) in the grating material that lie transversely to the longitudinal axis of the grating and in which the refractive index is either lower or higher than in a surrounding part of the grating and where the distance between mutually adjacent regions can be varied, wherein the method is characterised by causing said regions to have mutually the same width, and by determining the positions of the different regions along the longitudinal axis of the grating in relation to the wavelength to be reflected.
The invention also relates to a reflector of the kind that has the characteristic features set forth in claim 7.